The invention relates to a heater with electrical heating elements for waterbeds, which is arranged between a bed frame and a safety film, and which controls the temperature of a metal plate placed under the safety film of a waterbed core lying thereupon, wherein the heating elements are connected in a heat-conducting manner with the bottom side of the metal plate in a flat casing via a readily heat-conducting layer.
In a heater of this kind according to DE 195 08 315 C1, the electrical heating elements consist of electrical conductors made out of a pasty mixture of noble metal particles, e.g., gold, silver or ruthenium, and ceramic constituents, such as glass and aluminum oxides, which are burned in as a hybrid conductor loop on a ceramic plate consisting of an aluminum oxide substrate. When carrying a current, these electrical conductors generate heat, which is conveyed to the metal plate of the casing of this heater via the ceramic plates as the heat conductors and via the readily heat-conducting layer. The ceramic plates here do not act as heat generators, but only as pure heat conductors of the heat generated by the electrically burnt-in resistance conductor loops when energized. To this end, the heat-conducting layer with which the ceramic plates are bonded to the bottom of the metal plate consists of an adhesive with high heat conductivity. An NTC sensor is arranged on the bottom of the deflection-resistant metal plate, while the current conductors leading to the burnt-in resistance wire conductors are connected with a TRIAC, which is also attached to the bottom of the metal plate. Both the NTC sensor and TRIAC are connected with a controller.
According to DIN-EN 60335-2-66 of February 1996, the temperature on the surface of a waterbed heater cannot exceed 60xc2x0 C., while the temperature on the surface of the waterbed mattress cannot exceed 37xc2x0 C. Further, the temperature increase on the surface of the waterbed heater cannot exceed 125xc2x0 C.
The previously described waterbed heater according to DE 195 08 315 C1 cannot satisfy these requirements, since the NTC sensor is attached to the metal plate, as a result of which it only measures the metal plate temperature, and hence can only control the temperature of the metal plate. Such an NTC sensor operates according to the principle that its electrical resistance decreases as the temperature increases, and vice versa. As a consequence, a cable break causes the resistance to undergo an infinite increase, mistakenly indicating a low temperature, which relays the command to heat to the controller.
Further, this heater cannot accommodate various waterbed cores, i.e., waterbed cores with different volumes of water, frame composition (metal, foam or wood) and different covers, e.g., a covered and non-covered waterbed core, because the respectively differing heat radiation losses cannot be detected by the NTC sensor in light of the various aforementioned conditions. In this case, experience has shown that the temperature of the waterbed core deviates considerably form its desired control temperature. In addition, since the bottom of the metal plate is fitted with small ceramic plates as heat conductors, which take up only a small area of the metal plate, zones with highly disparate temperatures are present on the metal plate during the heating phase. As a result, its radiating surface exhibits a considerable temperature fluctuation. These temperature fluctuation is understood to mean temperature deviations on the surface of the metal plate in the form of temperature spikes directly above the ceramic conductor, and temperature valleys between the two ceramic conductors, which become evident in the heating phase.
In turn, this causes the NTC sensor to relay a temperature to the controller that does not coincide with the actual average temperature of the heating phase, but at most in the stationary state.
WO 98/36664 disclosed a waterbed heater in which a layer with a high electrical resistance is attached under the metal plate, and an electrical resistance wire heater thereupon, which is in turn shielded form the casing by a layer with a high electrical resistance. This heater has the same disadvantages as the one mentioned at the outset, specifically that the conductor loops can completely or partially tear given an undesirably high load and sag of the metal plate, which interrupts current conduction. This holds true all the more so since a layer of air or other dampening layer made out of soft material is frequently located between the electrical conductor loops and underlying casing floor for heat attenuation, facilitating a sag in the metal plate. A protective device that independently, and hence automatically, prevents the metal plate from overheating under all conditions given a failure of a control line leading to it can also not be encountered in this waterbed heater. This case involves no more and no less than a conventional resistance wire heater with all associated disadvantages that is known in numerous variations.
Proceeding from this most obvious prior art, the object of the invention is to provide a heater of the generic type mentioned at the outset, which, while avoiding the aforementioned disadvantages, consistently prevents the metal plate from overheating on the one hand, and ensures a reliable control of temperatures in the waterbed core at varying volumes of water for different frame compositions, whether metal, foam or wood, and given varying room temperatures and covers, and hence under different heat transmission conditions.
This object is achieved according to the invention in conjunction with the generic notion mentioned at the outset by having the heating elements consist of several current and heat conducting metal elements held together non-positively and/or positively by current carrying coupling elements, and NTC heating elements clamped in between that generate heat when energized, and by designing the readily heat-conducting layer between the bottom of the metal plate and heating elements as a bilaterally adhesive film layer with good current insulating properties.
For the first time, this design provides a waterbed heater with real ceramic heating elements, i.e., with those NTC heating elements that generate the desired heat directly when energized, and do not act simply as heat conductors of a burnt-in hybrid resistance conductor loop, as in prior art. Since the heating elements now consist of several current and heat conducting metal elements held together non-positively and/or positively by current carrying coupling elements, the entire bottom surface of the metal plate can be heated completely uniformly, without any noticeable fluctuations.
In addition, the NTC heating elements offer an automatic, and hence independently arising safety function, since its electrical resistance rises steeply at a temperature of 90xc2x0 C., for example, and approaches infinity at a temperature of 100xc2x0 C., so that the flow of current through the metal elements is interrupted, thereby precluding any overheating.
In an advantageous further development of the invention, the metal elements consist of aluminum or copper sections with a high mass, and hence a great storage capacity. The NTC heating element is advantageously made out of barium carbonate, titanium oxide and other additives, while the film layer consists of a permanently elastic heat conducting film filled with an acrylate adhesive.
The coupling element advantageously consists of a resilient contacting clamp, which non-positively, conductively and positively couples two balanced metal elements with the NTC heating elements lying in between. In this way, simple means are used to obtain an extremely compact overall heating element, which can do without any and all adhesive layers situated in between. To simplify the coupling of the contacting clamps, the side of the metal elements facing away from the current-insulating and heat insulating film are provided with dovetailed recesses for the positive engagement of contacting clamps.
In an advantageous further development of the invention, the heater has a total of four parallel running metal elements with a total of six NTC heating elements located in between, under the metal plate and film layer, as an overall heater.
In a particularly advantageous further development of the invention, the heater is controlled by two NTC sensor, of which a first is situated outside the area of the metal plate in an outside area of the casing in such a way as to be in direct measuring contact with the waterbed core, and a second is arranged under the metal plate in a known manner. As a result, the second NTC sensor always measures the temperature of the metal plate, and the first NTC sensor always measures the temperature of the waterbed core. Both NTC sensors are connected with a microcomputer, which continuously evaluates both temperatures, and controls the temperature of the metal plate in such a way that the water temperature of the waterbed core rises to a temperature adjustable with a controller. This ensures that the desired waterbed core temperature is not only always reached, but held constant, completely independent of the water volume level, the entirely different constitution of the waterbed frame, whether it consist of metal, foam or wood, and completely independent of the room temperature and cover states of the waterbed core. In this case, the microcomputer is set in such a way that the surface temperature of the metal plate does not exceed 60xc2x0 C. In addition, a malfunction is precluded by the microcomputer via a plausibility check given a break in one or both NTC sensors or line leading thereto, and hence both the temperature of the metal plate surface and temperature of the waterbed core are limited to the values permitted under the DIN standard.